Process for improved high silica fibers



March ."I'o,l 1970 R. R. SAF F ADI PROCESS FOR IMPROVED HIGH SILICA'FIBERS Filed July 18, 196e wmww MR United States Patent O 3,498,774 PROCESS FOR IMPROVED HIGH SILICA FIBERS Richard R. Saffadi, Ridgefield, NJ., assignor to J. P. Stevens & Co., Inc., New York, N.Y., a corporation of Delaware Filed July 1s, 1966, ser. No. 566,044 Int. Cl. C03c 25/00 U.S. Cl. 65-31 6 Claims ABSTRACT OF THE DISCLOSURE This invention pertains to a method for the formation of highly siliceous glass fibers whereby glass fibers are first leached to dissolve the acid soluble oxides, then washed, immersed in an ammonium hydroxide bath, and finally dried by heating in a furnace at 1650 F. to 2200" F., wherein the improvement comprises the use of a liquid ammonium.

As a result of such leaching treatments, glass fibers in the form of yarn, batts, cordage, fabric, etc., can be prepared which have an exceedingly high silica content, i.e., between about 99.0 and 99.9%. Such treated fibers can withstand very high temperatures without degradation and have therefore been found uniquely suited for the manufacture of insulation material.

For example, the leached high silica fibers are frequently utilized to prepare resin-silica laminates which are shaped to form insulating shields around an appropriate body. Thus, high silica fibers in the form of a fabric are often impregnated with a resin, such as a phenolic, polyester, or epoxy, and then built up in layers to form a strong heat-resistance laminate. The exceptional ablation characteristics of such resin-silica laminates make them particularly useful for the protection of the leading surfaces of very high-speed aircraft or missiles Where temperatures often exceed 2000 F. and frequently reach 5000 F. or more.

The extreme environmental conditions under which such resin-silica laminates are used require, in the first instance, stringent quality control. The ablative effects of the environment of use tends quickly to exploit any inherent weaknesses in the product. Experience has shown, in this respect, that the water content of the glass fiber, whether unbound or as water of hydration, has an adverse effect on the product. Products prepared from high silica fabric sometimes rupture or blister during use due to the release of Water vapor, thereby severely weakening their protective qualities. Moreover, high water content, either free or bound, tends to interfere with the application of resins to the glass, particularly when the high silica fabric has a relatively low pH, i.e., below about 4.0.

Aside from the above drawbacks of conventionally prepared high silica fibers, it is known that the leaching of the glass to remove the non-siliceous constituents tends to weaken the fibers. In order to maximize the physical strength of the leached fibers, it has been found necessary that the step of heating the acid-leached fiber be carefully controlled. During this heating operation, which rice is commonly done in a furnace at about 1650 F. to 2200 F., the strength of the ber changes depending upon the time of heating. Ordinarily the strength (measured after firing) of a leached glass fiber increases with firing tlme until a maximum value is reached, thereafter there occurs a significant decrease in strength, i.e., an inversion. Consequently, the risk of obtaining goods of less than maximum strength is appreciably enhanced unless close control of the heating cycle is maintained, which is not always possible.

It is a primary object of the present invention to overcome the drawbacks associated with the conventional process for acid leaching of silica fibers, and more particularly it is an object of the present invention to provide a method for preparing high silica fibers, whether as a fabric or otherwise, that are compatible with resins and which are suitable for the manufacture of ablation-resistant materials.

It is a further object of the present invention to provide a method for preparing high silica glass fibers of reduced moisture content and reduced Water of hydration, and of increased pH.

It is another object of the present invention to provide an improved method for obtaining high silica fibers of consistent high strength.

According to the invention, the high silica fiber is subjected to treatment in an ammonium hydroxide bath after the fiber has been leached but before it has been subjected to heating in the furnace.

The ammonium hydroxide bath has been found to give remarkably improved properties to the high silica fiber. In this respect it has been found that an ammonium hydroxide-treated silica fabric that has also been heat-treated and thoroughly conditioned with respect to relative humidity exhibits -a significantly reduced amount of absorbed water and water of hydration as compared to similarly prepared fabrics not treated with ammonium hydroxide. Further, the pH of a treated fabric is increased as compared to a conventional untreated fabric. As a consequence, the fibers of the treated fabric are more compatible with resins, particularly phenolics, and the products formed therefrom are less susceptible to blister and rupture when incorporated in objects subjected to high temperature service.

Equally significant is the finding that the ammonium hydroxide-treated fibers do not risk significant decreasing strength values during the heating cycle. That is to say, the improved effects can be obtained at lower furnace temperatures or shorter residence times than would be required for non-ammonium hydroxide-treated fibers. As a consequence, physical degradation of the silica fiber is impeded and the furnace does not have to work-as hard.

The ammonium hydroxide bath treatment of the present invention is a single step in the overall process of high silica fiber manufacture. It is preceded by the acid leaching step and generally a wash-water step that rinses excess acid and contaminants from the fiber. The fiber, usually in the form of a woven fabric, is then immersed in the ammonium hydroxide solution, either with or without a prior drying step.

After removing the fabric from the ammonium hydroxide solution, it is preferably washed with a Water spray to remove excess ammonium hydroxide, then dried at moderate temperature and thereafter heat-treated in the furnace to shrink the fabric and drive off as much water as possible.

The ammonium hydroxide bath is effective at room temperature, but slightly improved effects can be obtained at elevated temperatures, e.g. 1Z0-140 F. The concentration of the solution may be maintained in the range 3 of about 2 to 20% by weight of NH4OH, and preferably is maintained between about to 10%.

measurements were after conditioning at 50.5% i2%, R.H. at 74 F i2 F. for 16 hours.

TABLE II Furnace temp., Moisture 1 Water oi Sample NHO FJtime, content, hydration,

o. bath NH4OH bath conditions min. percent percent pH C No 1, 900/3 1. 09 0. 41 3.65 D No 1, 700/3 4. 86 1. 27 3. 60 E Yes.. 140 g./l (NH3) at 1Z0-140 F.. 1, 700/3 1. 24 0. 38 5. 30 F 2 Yes. 140 g./l (NH3) at 120-140 F.. 1, 700/3 1. 19 0.43 5.05 G Yes.. 140 g./1 (NH3) at room temp 1, 700/3 1. 76 0.68 4.00

Yes.. 140 g.ll (NH3) at room temp-. 1, 700/3 1. 55 0. 76 4. 30

1 Loss of moisture measured after 4 hours at 110 C.;|:2 C.

2 N ot dried; immersed wet.

The time of immersion of the silica fiber should be sucient to obtain substantial contact between the surface of the silica ber and the solution. Complete penetration or impregnation is therefore desired and the time required depends upon the physical nautre of the silica, i.e., whether it is yarn, batt, cordage or fabric, and upon the weight of the fiber. Most yarns can be adequately treated within about 5 to 10 minutes, for the concentrations of ammonium hydroxide referred to above.

EXAMPLE l Two samples of 241/2 oz./yd. fiberglass fabric having G filament yarn and a thread count of 44 warp and 35 ll were separately treated to form high silica materials. Both samples (identified as A and B) were first leached to dissolve the acid soluble oxides by immersion for 90 minutes in a concentrated bath (160 g./l.) of hydrochloric acid at a temperature of about 190 F. They were then washed with demineralized water to remove residual acid and solid contaminants.

Thereafter, Sample A was dried at 110 C. for 5 minutes, then fired in a furnace at 1700 F. for 3 minutes and finally conditioned at 65% relative humidity (RH.) at 72 F.

Sample B was treated, i.e., dried, fired and conditioned, in the same manner as Sample A except that, prior to the drying step, the B fabric was immersed for a few minutes in a room temperature ammonium hydroxide bath (70 g./I. of NH3).

Both the ammonium hydroxide treated Sample B and the non-treated Sample A were tested for percent by weight of moisture content (unbound), water of hydration, and pH. The results are given below in Table I.

TABLE I Sample A Sample B Moisture content, percent 8. 19 7. 08 Water of hydration, percent..- 2. 18 1. 05 pH 3. 55 4. 65

These results show that the ammonium hydroxidetreated fabric (Sample B) could take advantage of a lower firing temperature to yield a shrunk product with a lower water of hydration, a lower water content, and a higher pH, than an untreated fabric.

EXAMPLE 2 A comparison of Samples G and H with D shows that the moisture content of the fabric is reduced considerably, as is the water of hydration, when the leached fabric (wet or dry) is treated at room temperature according to t-he invention.

The elect is even more noticeable when the arnmonium hydroxide bath is maintained atbetween and F. (compare Samples E and F with Sample D).

A comparison of the results of Sample C with those 0f Samples E and F reveals that a treated fabric fired at 1700 F. for 3 minutes has a moisture regain and a water of hydration similar to an untreated fabric iiredat the higher 1900 F. temperature.

In all cases where the ammonium hydroxide treatment was used, the pH showed an appreciable increase as compared to untreated specimens, with the hot ammonium hydroxide bath showing the highest pH.

EXAMPLE 3 Specimens of all of the samples prepared in Example 2 were anayzed for rate of moisture regain.

After each specimen was conditioned as set forth in Example 2, it was heated for 30 minutes :1 -1 minute at 325 F. i5 F. and the rate at which the fabric returned to the equilibrium condition of moisture content was carefully measured. The results of these measurements are set forth in FIGURE 1 hereof, which is a semilogarithmic plot of percent moisture content against time.

From FIGURE 1 it is seen that at equilibrium the conditions were as follows:

Although Sample C (non-treated fabric lired at 1900 F.) showed the smallest moisture content and the fastest rate of moisture regain, the treated samples fired at 1700 F. (Nos. E, F, G, H) had far smaller moisture contents than the comparable untreated samples fired at 1700 F. (No. D), and were only slightly higher than Sample C. Again, among the treated samples the ones using the hot ammonium hydroxide (Nos. E and F) showed the lowest values.

The fabrics subjected to higher firing temperatures show appreciably reduced moisture content however, the 'higher furnace temperatures are also known to degrade the physical strengths of the fibers and thus are considered detrimental to the fiber. On the other hand, the ammonium hydroxide treatment of the invention acts to reduce moisture content and water of hydration without the necessity of going to strength-degrading firing temperatures.

5 EXAMPLE 4 A G-iilament high silica glass fabric or 241/2 oz./yd.2 and a thread count of 44 warp and 35 ll was prepared by leaching the fabric in 160 g./1. HC1 for one hour at 6 and dehydrating step, with a solution of ammonium hydroxide at a temperature ranging between room temperature and 140 F., whereby the amount of absorbed water and water of hydration is significantly reduced as cornpared to similarly prepared, non-ammonium hydrox-ide 190 F. and thereafter drying at 110 C. for 5 minutes. 5 Samples of this fabric were finished both with and withtreated brous glass out ammonium hydroxide treatment as set forth below in 2'.The proces accordlpg to Clal'm 1 Where1-n the am' Table III and then tested for moisture content, water momu'm hydroxlde solutlon has,a concentrfltlon of be' of hydration and pH, after conditioning at 65% R.H. tween about 2 and 20%.ammomu-m hydroxldf at 72 E for atleastz days 10 3. The process according to clalm 2 wherein the con- TABLE HI Furnace temp., Moisture Water of Sample NILOH F./time, content, hydration, N o. bath NHiOH bath conditions min. percent l percent 1 pH 1 I No 1, 700/3 8.19 2.13 3. 55 J Yes 70 g./1. (NH3) at room temp- 1, 700/3 7.08 1.05 4. 65 K N 1, SOO/3 7.11 1.86 3.50 L Yes 70 g./1. (NH3) at room temp- 1,800/3 5.80 1.18 4. 55

1 Average values.

A comparison of the test results of Samples I and I centration of the ammonium hydroxide is between about and of Samples K and L shows that the ammonium hyand 10%. droxide treatment effectively reduces both the moisture 4. The process according to claim 1 wherein the temcontent and water of hydration, with a consequent rise perature of the ammonium hydroxide solution is between in pH. Consequently, a leached fabric pre-treated with about 120 F. and 140 F. ammonium hydroxide will require a relatively low firing 5. The process according to claim 1 wherein the amtemperature to yield a well shrunk silica fabric with a monium hydroxide solution has a concentration of below water of hydration content and an acceptable pH. tween about 2% and 20% and a temperature of be- It is noteworthy that the increase in pH is not due tween about room temperature and 140 F., and the to residual NH3 in the silica matrix, for analyses have leached glass liber is immersed in said solution for beshown that the nitrogen content of treated and untreated tween about 5 and 10 minutes, silica fibers after firing are practically the same, ranging 6. The process according to claim 1 Where-in the from 20 to 70 ppm. 35 silica glass fiber product is a fabric.

The pH can be even further increased by subjecting the silica fabric to an atmosphere of ammonia vapor, as References Cited placing the fabric (after firing) OI' abCu't One-half P minute 1n the vapor space over a pan containing a concentrated ammonium hydroxide solution. 3,113,008 12/1963 Eimer 65-31 What Ihclaim is: f h h l l b 3,149,946 9/1964 Elmer 65-31 1. In t e rocess o preparing a ig si ica g ass er product coniprising the steps of acid-leaching a 4fibrous s' LEON BASHORE" Primary Exammer glass and thereafter heat-shrinking and dehydrating said JOHN H. HARMAN, Assistant Examiner brous glass at between about 1650 F. to 2200 F., the 45 improvement comprising contacting the fibrous glass, after the acid-leaching step and before the heat-shrinking U-S. C1 XQR. 156-14.

G J UNITED STATES PATENT OFFICE T CERTIFICATE 0F CORRECTION racen: No- 3,498,774 Dated MB-ICh 3,r 1970 Richard R. seffedi InventOl-(S) It :ls certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

V'In the Specification "i Column l line 26 delete "2,1#62 8&1" and insert therefor 2,l+6l,l-; Column l, rline [t5 delete "resistance" and insert therefor- -res1eta.nt. Column 3, line 23 delete "neutre" and insert therefor "nature". Column 4, line 39 delete "anayzed" and insert therefor --a.na.1yzed-.

SIGNED AND SEALED 

